The Catholic Church has opposed the practice abortion from its very beginning. One of our earliest sources on the beliefs and traditions of early Christians is a document called The Didache, or “The Lord’s Teaching.” It’s a brief, anonymous, Christian treatise written in Greek that we can reliably date to the first or second century AD. In it we find descriptions of the Eucharist, text of the Lord’s Prayer, teachings on baptism, and notably, a prohibition on abortion.

The document reads:

οὐ φονεύσεις τέκνον ἐν φθορᾷ οὐδὲ γεννηθέντα ἀποκτενεῖς

Taken literally, this has been translated to “You shall not murder a child in destruction nor shall you kill one just born.” This has been widely agreed upon by scholars to indicate a condemnation of abortion alongside infanticide.

Now we do see some changes in how the church has understood abortion and pregnancy throughout history, but it is clear that the practice has always been vehemently opposed. Thomas Aquinas, like many Christians of his time, believed that life did not begin until “quickening”, a term used to indicate when an unborn child behinds to kick and move. They believed that early abortion was not murder, but abortion after the quickening was. To be clear, they still opposed early abortion. The Catholic Church also holds a view of sexual intercourse as being sinful when contraceptives are used. Early abortion was viewed as contraceptive in nature, and was therefore still considered sinful.

In response to recent scientific developments, the Catholic view on abortion has evolved slightly. We know that human life begins at conception - from the moment the sperm and ovum fuse a new organism, with new DNA, is formed and begins to develop. This organism, by any elementary definition, is both alive and human. As such the Church now holds the view the abortion is sinful because it is murder, at any point in the pregnancy. For reference on the modern Catholic view, see the encyclical Evangelium Vitae from Pope St. John Paul II. Of particular interest to the question is the following quote:

procured abortion is the deliberate and direct killing […] of a human being […] extending from conception until birth.